SVX Engine cooling "Again & Again"

[Dessertrunner]

Tried pulling the fuses on the fans and reving the engine to see if the cooling is different between the heads. Bad news it didn't work as radiator just moved up at the same rate as the engine. There was not enough cooling to work. Even with the fans on it didn't work.
Tony

[FriendlyTurkey]

Originally Posted by Trevor

Air is dissolved in water at ambient temperature, to become released at about 80 deg. C or more, depending on pressure. This air remains within the closed system, unless bled out due to the cap opening to the overflow container and then not completely.



Bubbles of nothing?
Bubbles of water vapour/steam will form if the pressure at any point within the system, temporally exceeds the set radiator cap pressure whereby the water will boil. Such pressure hot spots can occur due to resistance to flow or blind sections, as per my previous posts. The essence of the problem with water vapour/steam, is not "temperature equalisation," which is a result, not a cause.



If were are such holes, the system could not hold the designed pressure. No outlets, including any at the rear of the head can bypass the radiator cap and bleed to the atmosphere. This is the very reason for careful slow filling, being required.

It must be concluded, that the only way of stopping water vapour being generated at any specific point, is to eliminate localised pressure.
P.S. As I now see yt Tom has diligently pointed out.

I agree that a excessive pressure drop somewhere in the system may be causing cavitation to occur. I see the same problem when sizing industrial controls valves at my work. Attached is a simple graph showing what happens when I size a valve with too much pressure drop for a given system.



Has anyone tried a 35psi pressure cap to give more headroom for this pressure drop? May not solve the problem, but mask it enough for some.

http://www.speedwaymotors.com/High-P...isplayId=13000

The major trouble with running the cooling system at elevated pressure is the reduction in flow from the pump.

FT

[shotgunslade]

I had dismissed the idea of local pressure drop cavitation because this is a closed loop, but I suppose that a local restriction which would require a large decrease in static pressure to accelerate the fluid to the required velocity through the restriction, which could conceivably drop the local pressure below the flash point of the fluid.

Assuming that the radiator cap isn't blowing, then the average system pressure is going to be below 2 Bar, depending where in the radiator cap is located in the line of pressure drop around the system. Let DP be the pressure drop through the whole circulation system; the amount of pressure the pump has to generate to get the flow. Assume the radiator cap is located halfway between (from a pressure drop standpoint) the pump inlet and the pump outlet. The absolute pressure at the pump outlet is going to be

(2 Bar + 1/2* DP).

The pressure at the pump inlet is going to be

(2 Bar - 1/2 * DP)


So, if the pressure drop through the system is 1.2 Bar, then the absolute pressure at pump inlet is going to be .9 Bar. If the water is a 220 DegF, it's boiling at that point, and the pump will lose flow, until the steam bubble passes or collapses and the pump rotor is fully immersed again.

This could happen with a centrifugal water pump operating well above its design RPM, developing more flow against a system pressure drop that increases with increasing flow.

So, we could have a negative suction head problem even in a closed system, when the fluid is operating at a temperature near its flash point.

[FriendlyTurkey]

I like your ideas shotgunslade! I hadn't had my cup of coffee yet! I didn't realize I was looking at a closed loop system with the pressure cap just being a safety valve for overpressure conditions.

It would really be of great value to measure pump inlet and outlet pressure!

[Trevor]

Originally Posted by shotgunslade

I had dismissed the idea of local pressure drop cavitation because this is a closed loop, but I suppose that a local restriction which would require a large decrease in static pressure to accelerate the fluid to the required velocity through the restriction, which could conceivably drop the local pressure below the flash point of the fluid.

Assuming that the radiator cap isn't blowing, then the average system pressure is going to be below 2 Bar, depending where in the radiator cap is located in the line of pressure drop around the system. Let DP be the pressure drop through the whole circulation system; the amount of pressure the pump has to generate to get the flow. Assume the radiator cap is located halfway between (from a pressure drop standpoint) the pump inlet and the pump outlet. The absolute pressure at the pump outlet is going to be

(2 Bar + 1/2* DP).

The pressure at the pump inlet is going to be

(2 Bar - 1/2 * DP)


So, if the pressure drop through the system is 1.2 Bar, then the absolute pressure at pump inlet is going to be .9 Bar. If the water is a 220 DegF, it's boiling at that point, and the pump will lose flow, until the steam bubble passes or collapses and the pump rotor is fully immersed again.

This could happen with a centrifugal water pump operating well above its design RPM, developing more flow against a system pressure drop that increases with increasing flow.

So, we could have a negative suction head problem even in a closed system, when the fluid is operating at a temperature near its flash point.

Sorry but your theory does not hold water.

The closed loop includes a radiator and the following does not normally apply.====

“So, if the pressure drop through the system is 1.2 Bar, then the absolute pressure at pump inlet is going to be .9 Bar. If the water is a 220 DegF, it's boiling at that point, and the pump will lose flow, until the steam bubble passes or collapses and the pump rotor is fully immersed again.”

[Trevor]

The pressure to be considered is in no way related to the atmosphere. The pressure within the circuit depends on the pump output as is opposed to the available flow path.

Increasing pump speed against a fixed resistance/head, increases the internal pressure. This will constitute a constantly reducing pressure gradient. However, because of huge variations in resistance throughout the flow path, this will not be consistent. The lowest pressure will finally occur at the pump inlet.

Flow will not increase in line with pressure. Pump speed, pressure, flow, will not increase in a linear fashion, far from it. In fact there is a final point whereby flow will choke at a maximum.

Taking the above and all of that mentioned in previous posts into account, there is a point where pressure becomes an absolute disadvantage. It is very possible that this point coincides with approximately 5,500 engine RPM.

Is there not strong motivation for a cheap and simple experiment involving reduced pump efficiency?

[shotgunslade]

The relationship with atmospheric is that the radiator cap will blow at some differential above atmospheric. The system is closed until that happens. The absolute pressure at the pump discharge may actually be above the level necessary to blow the radiator cap, but pressure drop between the pump and the cap may reduce it, so that the cap doesn't blow. If the absolute pressure at the pump inlet is below the flash point of the coolant, you will get cavitation.

If we know that the absolute pressure of the system at the radiator cap is below 2 Bar (the cap hasn't blown), we could calculate the pressure drop. If it is below the flash pressure of the coolant at whatever temperature the coolant is at that point, you will get boiling.

In general, system pressure drop follows the equation: (where DP = pressure differential, K is a consant and Q = flow)

DP = K * Q**2

So, yes, ieven though the system pressure drop increases with the square of the flow if we are running the engine way above the lnormal rev range, the pump will still have increased flow with increased rpm, even though it is only increasing at less than the square root of the rpm increase. Pressure drop across the ystem will be increasing drastically. Assuming the cap hasn't blown, the system flow pressure drop between the cap and the pump inlet could make the absolute pressure at teh pump inlet less than the flash pressure of the coolant at that point.

[dynomatt]

Remember the old cars with something called a fan clutch? They operated the mechanical radiator cooling fan on a hydraulic disc such that when it achieved a certain load (or rpm), it couldn't spin any faster...this sounds like something we should try with the water pump...above certain RPM's it becomes a fixed speed.

We can do this with electric pumps, but not with the mechanical one.

If this is the case, why have the people who have dabbled with electric pumps (I can think of a handful), not been able to achieve a solution?

I've procured a BMW water electric water pump in an effort to solve some of this.

Based on everything I've heard, my collection of ideas to solve this is:

1) Dual top outlets, separate from each other (like the current six cylinders)
2) Alloy radiator - (PWR or similar)
3) Electric pump
4) Gutted original pump to operate just like an idle pulley, but paying particular attention to flows in and around the original pump
5) Heater system removed - to avoid complications
6) Throttle body water heating removed - to avoid complications
7) Dual high air displacement fans
8) Engine oil cooler
9) Modified thermostat

When will I do this all...hopefully over our summer break...but slowly.

Cheers,
Matt

[yt]

Electric pumps have proven only to be a bandaid to the cooling issue. it makes it better but does not address the real issue at hand.

Tom

[Dessertrunner]

I agree with Tom at this point in time there is no evidence to suggest that the pump is the problem.
As regards pressure in the system, a higher pressure can only happen if part of the coolent system has a restriction that decreases the flow. Any potential pressure restriction is likly to be with in the engine which would mean that there is a potential vacum on the radiator side of the circut due to the pump suction. Having driven for the last week with the 3 temp guages there has to be a major problem with the flow in the left bank. Every time I look the two heads are showing near 10c difference. I agree with the general opinion that the next work needs to be on the top pipe.
The recent discussion leads me to think that I need to find a way to fit a pressure guage or is it correct to assume that Temp is flow. Is the current thought that the left and right heads have the same coolent pressure. I personally think they would be different due to the reduced flow between left and right but am open to correction.
Tony

[oab_au]

While I agree with Toms approach to use two pipes to a two inlet radiator, it may be beyond the possibility's of the average member. I agree it is best to keep using the std pump, till we find if removing the restriction will solve the problem, as most people would be using the std pump.

I think there is merit in Tony's center tap of the pipe, to remove the restriction and provide equal flow, as far as we can.

If the modded pipe was provided with a center divider to direct the flow from each side to the outlet pipe, each side would have the same resistance, and flow the same.
I bit like this.



May not be too hard to do .

Harvey.

[yt]

add a third small hole to each side... Open up a block and you will see why.

Also, remember making them equal on top means they are unequal as a system. it would be best to run both pipes on the RH side of the engine. The coolant has to flow into the block farther on the RH side so the make it as equal as possbile having a shorter exit would essential try to balance the two halves out

Tom

[oab_au]

Originally Posted by yt

add a third small hole to each side... Open up a block and you will see why.

Also, remember making them equal on top means they are unequal as a system. it would be best to run both pipes on the RH side of the engine. The coolant has to flow into the block farther on the RH side so the make it as equal as possbile having a shorter exit would essential try to balance the two halves out

Tom

Well I can only go as far as opening the hood. I have to leave it to you blokes to supply the inside details.

The bottom may be uneven, but it is the top outlet that is causing the main problem. If we can get that as even as we can to eliminate the problem there, we can see if there is a need to go further.

Harvey.

[yt]

by creating one leg longer than the other you are essentially causing less resistance in the shorter leg and we are back at step 1.

Tom

[Trevor]

Originally Posted by shotgunslade

The relationship with atmospheric is that the radiator cap will blow at some differential above atmospheric. The system is closed until that happens. The absolute pressure at the pump discharge may actually be above the level necessary to blow the radiator cap, but pressure drop between the pump and the cap may reduce it, so that the cap doesn't blow. If the absolute pressure at the pump inlet is below the flash point of the coolant, you will get cavitation.

If we know that the absolute pressure of the system at the radiator cap is below 2 Bar (the cap hasn't blown), we could calculate the pressure drop. If it is below the flash pressure of the coolant at whatever temperature the coolant is at that point, you will get boiling.

Yes the pressure at the pump inlet will obviously be less than the output. Also the input pressure can be close to the pressure as set by the radiator cap valve dependent on restriction within the radiator.

However at the pump inlet, the coolant has been cooled by the radiator so that at that point, under normal circumstances, the coolant will not be boiling. As previously pointed out, the radiator must be taken into account.

In general, system pressure drop follows the equation: (where DP = pressure differential, K is a consant and Q = flow)

DP = K * Q**2

So, yes, ieven though the system pressure drop increases with the square of the flow if we are running the engine way above the lnormal rev range, the pump will still have increased flow with increased rpm, even though it is only increasing at less than the square root of the rpm increase. Pressure drop across the ystem will be increasing drastically. Assuming the cap hasn't blown, the system flow pressure drop between the cap and the pump inlet could make the absolute pressure at teh pump inlet less than the flash pressure of the coolant at that point.

Although it is difficult to understand the exact point being made within the post, I certainly agree agree that “Pressure drop across the system will be increasing drastically.” as result of increased RPM. This is the very point, to which I have constantly been drawing attention.

However no one appears to appreciate the consequences of this issue, and it therefore remains ignored.